So took the output of the 100khz low pass filter coming off
multiplier, and multiplied it by 19k and -19k and took those outputs
as the left and right channels. Not what I expected. Both sides
"sound" the same (I put them through my fader into the sound sink) and
even putting the subcarrier frequency all over the map it doesn't
change, so I'm clearly not doing what I think I should be doing.

Below 19 kHz is the sum of the left and right audio channels. Above 19
kHz is the modulated difference of the left and right audio channels.
(The reason for the sum and difference thing is backwards compatibility
with mono receivers.) To separate the two signals you have to split
them after the FM demodulation step. You should be able to detect (and
synchronize to) a pilot tone found at 19 kHz. This looks like a good
description: